Hot melt adhesives are routinely used in various applications where a stable surface-to-surface bond must be formed. Further, hot melt adhesives are used in securing a variety of both similar and dissimilar materials together in a mating relationship, such as wood, plastics, corrugated films, paper, carton stock, metals, rigid PVCs, fabrics, leathers, and others. These adhesives are especially useful in applications where it is desirable to have the adhesive solidify rapidly after being dispensed.
One particularly significant area for use of hot melt adhesives is in package and carton sealing. Utilizing equipment generally known in the art, the hot melt adhesive is brought from a storage condition to an application condition and then applied to one or both surfaces of the materials which are to be mated in the package forming or carton sealing application.
One type of adhesive which has been traditionally used in package and carton sealing applications is based on ethylene vinyl acetate (EVA) and may be applied to the substrate, for example, by spray technique or by use of a hot gun.
Another material useful in package and carton sealing applications is a composition based on methyl methacrylate resins. Yet another alternative is a composition based on polyurethane resin (PUR).
In yet another variation, a multi-component adhesive composition intended for use in hot melt applications is comprised of several polymeric materials initially dispersed in a liquid or a low melting solid carrier.
This type of composition, generally of the type disclosed in patent application Publication No. U.S. 2004/0029980, by William C. Stumphauzer and Anthony A. Parker, is described as an adhesive composition containing multiple components. The composition is comprised broadly of a liquid phase, a reinforcing phase, an adsorbent phase, an activator, a thermal stabilizer, and optional secondary additives.
One of the challenges in preparing a multi-component adhesive composition containing the above functional components is substantially reducing or eliminating the tendency of the composition to “pack out”. This condition is observed, if at all, typically during the dispensing and application of the adhesive composition. Instead of readily flowing as a solid component dispersed within the liquid carrier, one (or more) of the solid components of the composition softens and tends to separate from the remaining components. As this component separates, it tends to block movement of the remaining dispersion components through the pumping apparatus, the dispensing head, or both. This tendency to interrupt movement of the remaining dispersion components can either hamper, or completely block, the flow. Further, the component which forms the bulk of the packed out material becomes physically separated from the remaining dispersion components, thereby altering the actual composition of the material being dispensed onto the substrate.
In the above multi-component adhesive composition, the adsorbent phase tends to interact with the liquid carrier phase as the storage condition temperature increases. As a result, the adsorbent phase tends to swell and become more sticky, thereby separating more readily from the other components of the adhesive composition, i.e., pack out.